Throughout this description, the expressions "liquid acid gas" or "acid gas in liquid form" will be used to designate the same product, namely acid gases which are present in a liquid state at the end of a processing step as opposed to a gaseous state.
The present invention is particularly well suited to processing a natural gas containing acid gases such as H.sub.2 S and/or CO.sub.2. In practice, it is often necessary to process this natural gas in order to remove at least some of the acid gases which it contains before transporting it.
The process most commonly used to separate these acid gases from the natural gas is one whereby the acid gases are absorbed in a solvent, as illustrated in the diagram of FIG. 1 and described below. The pressurised gas is brought into contact, in counter-flow, in a column CA with a solvent which is selective as regards the acid gases, which may be a chemical solvent such as diethanolamine (DEA) or methyldiethanolamine (MDEA), for example. The gas leaving the head of the column CA has had at least some of the acid gases, which are in a gaseous state, removed from it. The solvent leaving the base of the column CA is subjected to a first expansion process so that a gaseous fraction containing some of the methane co-absorbed from the natural gas can be removed in the separating flask SM, before being heated and expanded again in the exchanger E.sub.1 until a pressure close to atmospheric pressure is reached. The solvent is then regenerated in the distillation column CD, which is heated from the base by means of a reboiler RE. The gas leaving the head of the column CD is cooled in the exchanger DC by means of the ambient cooling fluid, for example water or the air. At the output of the exchanger DC, a separating flask DB separates a liquid fraction containing solvent and water, which is then delivered by means of a pump P.sub.2 to the head of the column CD, and a gaseous fraction enriched with acid gases which is produced at low pressure and which is close to water saturation. The solvent phase leaving the base of the column CD is picked up by the pump P.sub.1, cooled in the exchanger E.sub.1 then in the exchanger RA by means of the ambient cooling fluid, for example water or the available air, and recycled to the head of the absorption column CA.
Another known approach is to use a physical solvent, for example dimethyl-ether-tetraethylene-glycol (DMETEG), which can be regenerated, as illustrated in FIG. 2, by a series of expansions to a pressure close to atmospheric pressure. Consequently, gaseous fractions enriched with acid gases are obtained at the output of the separator flasks S.sub.1, S.sub.2 and S.sub.3 at decreasing pressure levels and are evacuated respectively via ducts 1, 2, 3. The fraction collected at the highest pressure from duct 1 contains a relatively high proportion of methane. It can be compressed and recycled to the inlet of the column CA or used as fuel gas.
The method illustrated in FIG. 2 is fine as long as the acid gases contained in the gas to be treated do not need to be completely removed, for example if the acid gas to be removed is CO.sub.2. In this case, the solvent can be regenerated by a simple process of expansion without heating.
In the two examples illustrated in FIGS. 1 and 2, the acid gases separated are in gaseous form.
The acid gases are evacuated to a location which will depend on their nature. Usually, CO.sub.2 is discharged directly to the atmosphere. Very toxic acid gases, such as H.sub.2 S, can not be discharged directly to the atmosphere and will have to be treated in a thermal installation by incineration, for example. The SO.sub.2 discharged from this processing may then be discharged to the atmosphere. Another approach is to deliver the separated H.sub.2 S to a Claus unit so that it can be converted into sulphur. However, conversion is expensive and not very practical on economic grounds.
In all cases, the standards and conditions which need to be observed for reasons of environmental protection are becoming increasingly stringent and the discharge of acid gases such as SO.sub.2 or CO.sub.2 directly to the atmosphere in gaseous form is becoming less accepted. It has therefore become necessary to find another method of processing which will avoid having to discharge directly to the atmosphere and one which will be acceptable in terms of cost.
One solutions is to re-inject these acid gases into the subsoil where they can be stored rather than evacuating or discharging them into the atmosphere. Under certain circumstances, this re-injection can improve recovery of hydrocarbons, particularly where petroleum and gas are being produced simultaneously at the re-injection location.
For re-injection, however, these acid gases have to be re-compressed at a pressure generally in excess of 10 MPa, which is expensive in terms of capital investment and high on operating costs. Furthermore, compressing acid gases with a high concentration of H.sub.2 S poses technological problems which are difficult to overcome due to the risk of leaks.